Eyes were on Vincenzo Nibali (Bahrain-Merida) and Chris Froome (Team Sky) at the stage finish but it was the Irishman who jumped clear to gain 29 seconds and move to within a fraction of a second to second place overall Esteban Chaves (Orica-Scott).
"That move was definitely not planned," he told Cycling Weekly spinning away on his trainer after the stage.
His jersey showed the grit and effort of the day, his smile revealed his satisfaction. "I thought why not give it a go and see what happens?"
He and Chaves now sit at 36 seconds behind Froome in the overall.
Nibali's Bahrain-Merida team made the pace on the Bermejo climb that topped out 21.8 kilometres to race. As expected, Nibali fired off on the descent.
Roche, who sat third overall at 1-05 minutes behind Froome at the start of the day, remained alert, chasing down Nibali before going on the attack himself.
"I knew Nibali was going to try it and he had his team making a very severe pace there on the climb. As expected, he attacked with about 600 metres to go on the hard bit. I managed to quickly get across. I was in a good position," added Roche.
Watch: Vuelta a España stage 10 highlights
"When I initially went, my plan wasn't to drop everybody but I could see some of the other GC guys far behind."
Roche's move caught the television motorbike by surprise, and while the television showed Froome speaking with Nibali, Roche was powering away.
He finished at 4-03 minutes and heard the line announcer struggling to understand what happened and which BMC rider was appearing.
The group zipped over the line at 4-32. In there were Roche's rivals like Michael Woods (Cannondale-Drapac), Adam Yates (Orica-Scott), Ilnur Zakarin (Katusha-Alpecin) and Wilco Kelderman (Team Sunweb) – places eighth though 11th overall. Roche's American team-mate Tejay van Garderen (BMC Racing) sits fifth overall at 1-27.
"Today's move was important for me as we're looking head to the next few days in the high mountains. Every second is very important and so if I could dig deep into the downhill for a few seconds on those other guys further down because you can see everybody's very close," Roche added.
"Yes, Vincenzo and Alberto and, if I think I can call them direct rivals then Wilco Kelderman, Michael Woods and Ilnur Zakarin. But we haven't gone up any real mountains yet and I'm not going to get over excited. I managed to get well around these early steep climbs and tomorrow starts the next test."
After a week of short and steep climbs, the Vuelta heads high tomorrow. Stage 11 climbs 15.5 kilometres to the Calar Alto observatory at 2120 metres.
"I traditionally like the two kilometre climbs at 20% and not four kilometre ones at 20%. And the same for the other climbs, seven to eight kilometres at 6%, not the 15km climbs at 12%!" Roche added.
"This year, every climb that they put in the Vuelta is a climb that I have to try to work my way around. I want to see how it goes. Like I said I'm in great condition now and it's just about staying focused."
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